Near-Field Multi-Layer Reflector for LED Light Extraction
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Solution Overview
Problem
Conventional semiconductor light-emitting devices suffer from low photon extraction efficiency, requiring numerous round trips and high optical loss, which increases cost and complexity, and fail to effectively direct a significant fraction of emitted light within an escape cone.
Innovation Solution
A semiconductor light-emitting device with a multi-layer reflector (MLR) structure positioned in near-field proximity to the active layer, utilizing dielectric layers of varying refractive indices to enhance photon extraction efficiency by reducing internal reflections and increasing the fraction of light propagating within an escape cone, accompanied by optical scattering elements to further redirect laterally propagating light perpendicularly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional semiconductor light-emitting devices are used, then the device structure is simple, but the photon extraction efficiency is low due to large refractive indices and total internal reflection
Solution Approach 1:
A multi-layer reflector (MLR) structure comprising alternating layers of dielectric materials with different refractive indices is positioned in near-field proximity to the active layer. This intermediary structure mediates between the high-index semiconductor and the low-index external medium, enabling efficient photon extraction by reflecting light back into the escape cone while minimizing absorption losses that would occur with metal reflectors.
Solution Approach 2:
The patent changes the optical parameters of the device by introducing dielectric layers with specific refractive indices and thicknesses. The MLR structure is designed with layer thicknesses of approximately λ0/4n (where λ0 is the vacuum wavelength and n is the refractive index), creating constructive interference for light extraction. This parameter optimization enables the device to achieve high photon extraction efficiency without requiring complex geometric modifications.
2Device complexity
If conventional reflectors are used, then the device complexity is low, but the number of internal redirections and reflections per photon is high
Solution Approach 1:
The MLR structure acts as an intermediary that redirects light more efficiently than conventional single-layer or metal reflectors. By positioning the MLR in near-field proximity to the active layer, photons are reflected back into the escape cone after fewer bounces, reducing the mean total number of internal redirections from tens of bounces to just a few, thereby reducing the time photons spend trapped in the device.
Solution Approach 2:
The reflector structure uses composite materials consisting of multiple dielectric layers with different refractive indices (e.g., alternating high-index and low-index dielectric materials). This composite structure creates distributed Bragg reflection, which provides high reflectivity for specific wavelength ranges while maintaining low absorption losses, thereby reducing the number of internal reflections needed to extract photons efficiently.
3Ease of manufacture
If the MLR structure is positioned far from the active layer, then the device manufacturing is easier, but the Purcell factor and photon extraction efficiency are reduced
Solution Approach 1:
The patent optimizes the positioning parameter by placing the MLR structure in near-field proximity to the active layer, specifically at a distance of approximately λ0/4n or less (where λ0 is the vacuum wavelength and n is the refractive index). This precise parameter control enhances the Purcell factor and photon extraction efficiency. The near-field positioning creates strong evanescent field coupling between the active layer and the MLR, which significantly boosts light extraction without requiring excessive manufacturing precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The MLR structure enhances photon extraction efficiency, reduces the number of internal redirections, and increases the fraction of emitted light within the escape cone, resulting in improved overall efficiency and reduced optical loss.
Implementation Method 1
The MLR structure reflects output light incident thereon from within the second semiconductor layer
Implementation Method 2
The MLR structure includes layers of dielectric materials of two or more different refractive indices
Implementation Method 3
such scattering can be induced by, e.g., diffraction from scattering elements arranged periodically or quasi-periodically within the MLR structure
Implementation Method 4
A plurality of optical scattering elements arranged within one or more layers of the MLR structure. The scattering elements can scatter laterally-propagating output light to propagate perpendicularly
Implementation Method 5
Near-field proximity of the MLR structure to the active layer, and structural arrangement of the MLR structure, can result in the device exhibiting one or more of: (i) a relatively enhanced Purcell factor
Data Source
AI summary
A light-emitting device includes a semiconductor diode structure and a multi-layer reflector (MLR) structure. The diode structure includes first and second doped semiconductor layers and an active layer between them; the active layer emits output light at a nominal emission vacuum wavelength λ0 to propagate within the diode structure. The MLR structure is positioned against a back surface of the second semiconductor layer, includes two or more layers of dielectric materials of two or more different refractive indices, reflects incident output light within the diode structure, and is in near-field proximity to the active layer relative to λ0. At least a portion of the output light, propagating perpendicularly within the diode structure relative to a device exit surface, exits the diode structure as device output light. The MLR structure can include scattering elements that scatter some laterally propagating output light to propagate perpendicularly.